Enhanced yeast expression using regulatory control sequences from yeast sorbitol dehydrogenase gene

ABSTRACT

An isolated DNA segment is disclosed which is derived from a Saccharomyces cerevisiae sorbitol dehydrogenase gene and which functions to increase expression of an associated foreign polypeptide when the DNA segment and the gene coding for the foreign polypeptide are operably linked in a vector in such a manner that the vector is replicated and carried by a host yeast cell. The functionally active portion of the segment is under the control (i) a transcriptional regulatory sequence of the sorbitol dehydrogenase gene of Saccharomyces cerevisiae, (ii) a translation initiation regulatory sequence of the sorbitol dehydrogenase gene of Saccharomyces cerevisiae, and (iii) a termination regulatory sequence of the sorbitol dehydrogenase gene of Saccharomyces cerevisiae. Yeast cells containing these regulatory sequences linked to a foreign DNA sequence are grown in medium containing sorbitol under conditions permitting a foreign polypeptide to be expressed.

FIELD OF THE INVENTION

The present invention is directed to recombinant DNA technology utilizing yeast host systems. In particular, the present invention is directed to a nucleotide sequence or fragment from a yeast sorbitol dehydrogenase gene which is capable of providing increased expression of foreign proteins in yeast.

BACKGROUND OF THE INVENTION

Developments in recombinant DNA technology have enabled cloning and expression of foreign genes in yeast. The utility of yeasts such as Saccharomyces cerevisiae as hosts for expressing mammalian and other foreign polypeptides offers advantages lacking in more commonly used prokaryotic hosts such as Escherichia coli. For example, yeasts are capable of glycosylation while prokaryotic hosts are not. The genetic system of S. cerevisiae is well-characterized, as are the principles for controlling gene expression. Additionally, S. cerevisiae is generally approved by the Food and Drug Administration.

The great practical and economic utilities of the yeast host system have been the impetus to attempt to maximize yeast expression by identifying various promoter regions to direct transcription, various sequences to terminate transcription, and other regulatory sequences. For example, yeast alcohol dehydrogenase has been identified as having a single strong promoter which is highly useful in enabling the attainment of substantial levels of expression for a variety of genes in yeast [Hitzeman, et al., Nature, 295: 717-722 (1981)]. U.S. Pat. No. 5,139,936 discloses a cloning vector containing a foreign gene and the yeast galactosidase (GAL1) regulatory region and promoter in position to increase expression of the foreign gene. U.S. Pat. No. 5,089,398 discloses the use of the promoter region from the glyceraldehyde-3-phosphate dehydrogenase to control expression of a foreign polypeptide in yeast. Thus, it is capable of providing which permits the attainment of substantial levels of expression for a variety of genes in yeast.

SUMMARY OF THE INVENTION

According to the present invention, it has been unexpectedly and surprisingly discovered that the promoter, terminator, and certain other regulatory regions from the yeast sorbitol dehydrogenase ("SDH") gene can be incorporated into expression vectors to significantly increase yields of foreign polypeptide in yeast.

In particularhe present invention provides a DNA segment isolated from a Saccharomyces cerevisiae sorbitol dehydrogenase gene which is utilized to increase production of a heterologous polypeptide when the DNA segment and gene encoding the heterologous polypeptide are operably linked in a vector whereby the vector is replicated and carried by a host yeast cell. The functionally active portion of the DNA segment is under the control of transcriptional and translation initiation and termination regulatory sequences of the sorbitol dehydrogenase gene.

The present invention further provides expression vectors capable of expression in yeast of a gene encoding a heterologous polypeptide. In particular, a vector of the present invention comprises DNA sequences under the control of transcriptional and translation initiation and termination regulatory sequences of the sorbitol dehydrogenase gene of Saccharomyces cerevisiae and a yeast replication system.

According to one embodiment of the present invention, a heterologous polypeptide is expressed in yeast by introducing a regulatory nucleotide segment from a Saccharomyces cerevisiae sorbitol dehydrogenase gene, which is induced by sorbitol, operably linked to a gene encoding a heterologous polypeptide in a vector whereby the vector is replicated and carried by the cell and the polypeptide expressed under inducing conditions.

According to another embodiment of the present invention, Saccharomyces cerevisiae is transformed with an expression vector comprising a DNA sequence under the control of transcriptional and translation initiation and termination regulatory sequences of the sorbitol dehydrogenase gene of Saccharomyces cerevisiae and a yeast replication system.

The present invention still further provides a DNA molecule comprising the sequence of FIG. 1, or degenerate equivalents thereof, wherein the polypeptide coded for by the DNA molecule has the biological activity of yeast sorbitol dehydrogenase. A substantially purified and isolated sorbitol dehydrogenase activity from Saccharomyces cerevisiae. is also provided.

Additionally, the present invention provides an assay reagent and an improvement in various assay methods known in the art which use heterologous polypeptides as reagents, or for the preparation of reagents, such as for the production of antibodies thereto. The heterologous polypeptide is produced by yeast cells transformed with a cloning vector which has a DNA insert under the control of a transcriptional sequence, a translation initiation sequence and a termination regulatory sequence of the sorbitol dehydrogenase gene. A test kit for use in detecting the presence of a member of a specific binding pair in a test sample is also provided which test kit comprises a container containing at least one SDH-expressed heterologous polypeptide or specific binding member thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (SEQ ID NO:1) illustrates the nucleotide sequence of the sorbitol dehydrogenase gene with translation of the open reading frame. Nucleic acid number 1-771 denote the transcriptional regulatory unit. Nucleic acid numbers 1846-2760 denote the transcriptional termination unit.

FIG. 2 is a schematic representation illustrating the construction of plasmids pxS3, pxS4, pxS4 linker. These vectors have a multiple cloning site flanked by promoter and terminator sequences of the SDH gene. Polypeptides are expressed as unfused molecules in pxS2, pxS3 and pxS4. The nucleotide sequence of the Multiple Cloning Site and Terminator of pAS2 is SEQ ID NO:9. The nucleotide sequence shown of the MCS of pXS4Linker is formed by insertion of the adaptor DNA comprising SEQ ID NO:7 and SEQ ID NO:8.

FIG. 3 is a schematic representation illustrating the construction of pxS3 containing the gene for hepatitis delta antigen

FIG. 4 illustrates the Western Blot analysis of hepatitis delta antigen expression in transformants containing pxS3 delta. Cell extract from transformants containing pxS3, was electrophoresed in lane 1 on SDS PAGE. Lysates from 2 individual transformants containing pxS3 delta were loaded in lanes 2, 3. Analysis was done by Western Blot analysis using human serum containing antibody to the delta antigen.

FIG. 5 is a schematic representation illustrating the construction of pxS2-SOD and pxS4 linker SOD.

FIG. 6 illustrates the expression of unfused human superoxide dismutase polypeptide in yeast transformed with pxS2-SOD. Expression of human SOD polypeptide was determined visually by staining with Coomassie Blue. Lane 1 contains high Molecular weight protein markers from Bio-Rad. Lanes 2,3 contain extracts from 8000-8B transformants containing pxS4 and pxS2-SOD respectively.

FIG. 7 illustrates the SDS-PAGE analysis of the expression of fused human superoxide dismutase polypeptide in transformants containing pxS4 linker SOD. The gel was stained with Coomassie Blue. Lane 2 contains extracts from transformant containing the yeast expression vector pxS4 linker. Two individual transformants containing the human SOD gene in pxS4 linker were electrophoresed in lanes 3,4. High molecular weight protein standard supplied by Bio RAD was loaded in lane 1.

DETAILED DESCRIPTION OF THE INVENTION

A. Definitions

The following terms are defined as used herein:

"Biological activity" of sorbitol dehydrogenase refers to a polypeptide having preferential oxidizing specificity for sorbitol as substrate.

"DNA expression vector" is any autonomous element capable of replicating in a host independently of the host's chromosome, after additional sequences of DNA have been incorporated into the autonomous element's genome.

"Gene" is a segment of DNA, a portion of which codes for a specific polypeptide or RNA molecule.

"Heterologous polypeptide" is a protein that is not normally produced by or required for viability of a yeast organism. This term contemplates the functional insertion of DNA encoding such polypeptide, employing recombinant DNA technology, into an expression vehicle, which in turn is used to transform a yeast organism host. Functional insertion of DNA denotes the insertion of DNA encoding the heterologous polypeptide into an expression vector under control of a promoter system, as defined below. The heterologous polypeptide according to the present invention includes, but is not intended to be limited to, hormones, such as human growth hormone, bovine growth hormone, leutinizing hormone (LH), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and the like; lymphokines; enzymes, such as superoxide dismutase, creatinine kinase MB (CK-MB) and the like; interferons, such as human fibroblast and the like; viral antigens or immunogens, such as foot and mouth disease antigens, influenza antigenic polypeptide, hepatitis delta antigen, hepatitis B core antigen, hepatitis surface antigen, hepatitis B e antigen, human immune deficiency virus 1 and 2 (HIV 1 and 2); human T-cell leukemia virus 1 and 2 (HTLV) and the like; and various other polypeptides, such as rennin, human serum albumin, human insulin, human insulin-like growth factor-1, various glycoproteins, and the like. The heterologous polypeptide according to the present invention can be used in pharmaceutical compositions, such as vaccines and the like, to treat and/or prevent affective disorders.

"Promoter" is a DNA sequence generally described as the 5' region of a gene, located proximal to the start codon. At the promoter region, transcription or expression of an adjacent gene is initiated. This is referred to as the transcription initiation site. At the promoter region may be a sequence of nucleotides that interacts as a control over the expression of any operably linked structural gene or genes.

"Operably linked" is the control exerted by the promoter over the initiation of expression of the polypeptide encoded by a structural gene.

"Transcription initiation site" is a DNA sequence of a promoter to which RNA polymerase binds, thereby initiating transcription of succeeding codons in a 5' to 3' direction.

"Transcription terminator" is a DNA sequence at the end of the transcript that causes RNA polymerase to terminate transcription.

B. Yeast Expression Vector

According to the present invention, a particular DNA segment is linked to a gene foreign to the yeast genome and incorporated into a modified strain of S. cerevisiae so that it produces a polypeptide under the control of transcriptional and translation initiation and termination regulatory sequences of the sorbitol dehydrogenase gene of S. cerevisiae. Expression is achieved by transformation of S. cerevisiae with a novel recombinant DNA expression vector.

In particular, a yeast expression vector is provided comprising (i) a yeast selective marker, (ii) a yeast origin of replication and (iii) yeast promoter and terminator sequences of the sorbitol dehydrogenase gene of S. cerevisiae positioned relative to a unique restriction site in such a way that expression may be obtained of a polypeptide coding sequence inserted at the restriction site. In particular a non-fusion vector cassette and a fusion vector cassette containing eleven amino acids from the amino terminus of the sorbitol dehydrogenase polypeptide have been constructed. Both cassettes have been inserted into a 30 copy yeast plasmid containing the yeast TRP1 gene as a selectable marker and 2 micron origin of replication. Fusion with eleven amino acids of the sorbitol dehydrogenase polypeptide results in several fold higher levels of expression of the fused polypeptides as compared to the unfused polypeptide. In addition, expression levels of heterologous polypeptides with sorbitol dehydrogenase as a fusion partner are several fold elevated over that of sorbitol dehydrogenase itself when expressed in multi-copy plasmids. Such polypeptides provide reagents for assays which are safer and more cost effective than lysates produced from the native organisms.

Any yeast replication origin known in the art may be used to construct the vector. For example, the replication region of the natural yeast plasmid 2 micron is described by the present invention. This plasmid is cryptic in that it confers no readily detectable phenotype and is present in about 100 copies per cell.

The SDH promoter of the present invention is preferably about a 771 base pair sequence from the yeast genome which contains signals for transcription of the SDH gene into mRNA and subsequent translation of the MRNA. Although the coding sequence for sorbitol dehydrogenase is not present in this DNA fragment, the fragment can direct the expression of foreign genes and the regulation follows the mode for the SDH gene.

The SDH terminator of the present invention is preferably about a 913 nucleotide sequence which includes the poly A addition site AATAAA.

The present invention provides for the expression of fused or unfused polypeptides from yeast. There is an unique restriction site XbaI at the 5' end of the gene within the coding sequence. Insertion of DNA sequences at this site results in the production of hybrid proteins containing the N-terminal eleven amino acids of the sorbitol dehydrogenase polypeptide fused to the amino acids coded for by the inserted DNA. Genetic information contained within this sequence that may confer high level expression of sorbitol dehydrogenase results in an increased level of expression of the fusion protein.

The yeast strain of the present invention is S. cerevisiae, a common laboratory strain of yeast used for its low toxicity and well known genetic characteristics. This strain is readily cultivated on a large scale. The recombinant DNA material of the present invention containing a DNA segment is under the control of transcriptional and translation initiation and termination regulatory sequences of the sorbitol dehydrogenase gene and is used to express a polypeptide product in any yeast cell capable of transformation, including, but not limited to, yeast mutants that alter regulation, and the like.

The vast majority of yeasts can be cultivated under relatively uniform conditions utilizing common laboratory media and methods known in the art. As would be understood by one skilled in the art, the typical growth requirements of yeast comprise an organic carbon compound for carbon and energy, organic or inorganic nitrogen for the synthesis of polypeptides and nucleic acids, various minerals, and a mixture of vitamins. Such growth requirements are met by yeast nitrogen base (YNB), a chemically defined medium which contains a number of trace elements, vitamins, trace amounts of amino acids to stimulate growth, and the principal minerals potassium phosphate, magnesium sulfate, sodium chloride, and calcium chloride. The nitrogen source is ammonium sulfate. The desired carbon source is added at a concentration of from between about 0.5% and between about 3%. The pH range of the medium is usually from between about pH 3.0 and about pH 8.0, preferably from between about pH 4.5 and about pH 6.5.

Microorganisms prepared by the genetic engineering processes described herein are exemplified by cultures now on deposit with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Md., and are identified as follows:

Accession Number 69249, Strain Escherichia coli, deposited Mar. 9, 1993;

Accession Number 69250, Strain Escherichia coli, deposited Mar. 9, 1993;

Accession Number 69251, Strain Escherichia coli, deposited Mar. 9, 1993; and

Accession Number 69252, Strain Escherichia coli, deposited Mar. 9, 1993;

C. Assay Formats

The present invention provides an improvement to methods for detecting antigens or antibodies in a specimen from an individual, wherein a recombinant antigen employed in the method is expressed as a fused or unfused polypeptide by a vector in yeast and wherein the expression of the recombinant antigen is under the control of a DNA segment from a Saccharomyces cerevisiae sorbitol dehydrogenase gene which increases expression of the gene coding for the polypeptide.

An assay reagent comprising an SDH-expressed heterologous polypeptide, or specific binding member thereof is provided. Preferably, the SDH-expressed heterologous polypeptide can be an enzyme. Most perferably, the SDH-expressed heterologous polypeptide can be superoxide dismutase. The assay reagent of the present invention can be used in any diagnostic assays known in the art. The assay preferably is performed as an immunoassay, although the present invention is not limited to immunoreactive assays. Any assay utilizing specific binding members can be performed. A "specific binding member," as used herein, is a member of a specific binding pair. That is, two different molecules where one of the molecules through chemical or physical means specifically binds to the second molecule. Therefore, in addition to antigen and antibody specific binding pairs of common immunoassays, other specific binding pairs can include biotin and avidin, carbohydrates and lectins, complementary nucleotide sequences, effector and receptor molecules, cofactors and enzymes, enzyme inhibitors and enzymes, and the like. Furthermore, specific binding pairs can include members that are analogs of the original specific binding member, for example, an analyte-analog. Immunoreactive specific binding members include antigens, antigen fragments; antibodies and antibody fragments, both monoclonal and polyclonal; and complexes thereof, including those formed by recombinant DNA methods.

"Analyte," as used herein, is the substance to be detected which may be present in the test sample. The analyte can be any substance for which there exists a naturally occurring specific binding member (such as, an antibody), or for which a specific binding member can be prepared. Thus, an analyte is a substance that can bind to one or more specific binding members in an assay. "Analyte" also includes any antigenic substances, haptens, antibodies, and combinations thereof. As a member of a specific binding pair, the analyte can be detected by means of naturally occurring specific binding partners (pairs) such as the use of intrinsic factor protein in the capture and/or indicator reagents for the determination of vitamin B₁₂, or the use of a lectin in the capture and/or indicator reagents for the determination of a carbohydrate. The analyte can include a protein, a peptide, an amino acid, a hormone, a steroid, a vitamin, a drug including those administered for therapeutic purposes as well as those administered for illicit purposes, a bacterium, a virus, and metabolites of or antibodies to any of the above substances.

"Solid phases" ("solid supports") are known to those in the art and include the walls of wells of a reaction tray, test tubes, polystyrene beads, magnetic beads, nitrocellulose strips, membranes, microparticles such as latex particles, and others. The "solid phase" is not critical and can be selected by one skilled in the art. Thus, latex particles, microparticles, magnetic or non-magnetic beads, membranes, plastic tubes, walls of microtiter wells, glass or silicon chips and sheep red blood cells are all suitable examples. Suitable methods for immobilizing peptides on solid phases include ionic, hydrophobic, covalent interactions and the like. A "solid phase", as used herein, refers to any material which is insoluble, or can be made insoluble by a subsequent reaction. The solid phase can be chosen for its intrinsic ability to attract and immobilize the capture reagent. Alternatively, the solid phase can retain an additional receptor which has the ability to attract and immobilize the capture reagent. The additional receptor can include a charged substance that is oppositely charged with respect to the capture reagent itself or to a charged substance conjugated tot he capture reagent. As yet another alternative, the receptor molecule can be any specific binding member which is immobilized upon (attached to) the solid phase and which has the ability to immobilize the capture reagent through a specific binding reaction. The receptor molecule enables the indirect binding of the capture reagent to a solid phase material before the performance of the assay or during the performance of the assay. The solid phase thus can be a plastic, derivatized plastic, magnetic or non-magnetic metal, glass or silicon surface of a test tube, microtiter well, sheet, bead, microparticle, chip, and other configurations known to those of ordinary skill in the art.

The test sample can be a mammalian biological fluid such as whole blood or whole blood components including red blood cells, white blood cells including lymphocyte or lymphocyte subset preparations, platelets, serum and plasma; ascites; saliva; stools; cerebrospinal fluid; urine; sputum; trachael aspirates and other constituents of the body which may contain or be suspected of containing the analyte(s) of interest. The test sample also can be a culture fluid supernatant, or a suspension of cultured cells. Mammals whose body fluids can be assayed for antigen analyte or antibody analyte according to the present invention include humans and primates, as well as other mammals who are suspected of containing these analytes of interest. It also is contemplated that non-biological fluid samples can be utilized.

The assay reagent of the present invention includes an indicator reagent comprising a label conjugated to a specific binding member of each analyte. Each indicator reagent produces a detectable signal at a level relative to the amount of the analyte in the test sample. In a preferred embodiment, each indicator reagent, while comprising a specific binding member of a different analyte, is conjugated to the same signal generating compound (label), which is capable of generating a detectable signal. In general, the indicator reagent is detected or measured after it is captured on the solid phase material. In the present invention, the SDH-expressed polypeptide includes one or more components of the indicator reagent(s). It is contemplated that different signal generating compounds can be utilized in the practice of the present invention. Thus, for example, different fluorescent compounds could be utilized as the signal generating compounds, one for each indicator reagent, and detection could be determined by reading at different wavelengths. Or, a short-lived chemiluminescent compound such as an acridinium or phenanthrindium compound and a long-lived chemiluminescent compound such as a dioxetane can be utilized to generate signals at different times for different analytes. Methods which detail the use of two or more chemiluminescent compounds which are capable of generating signals at different times are the subject matter of co-pending patent application U.S. Ser. No. 636,038, which enjoys common ownership and is incorporated herein by reference. Acridinium and phenanthridinium compounds are described in co-pending U. S. patent application Ser. No. 07/271,763 filed Jun. 23, 1989, which enjoys common ownership and is incorporated herein by reference.

In addition to being either an antigen or an antibody member of a specific binding pair, the specific binding member of the indicator reagent can be a member of any specific binding pair, including either biotin or avidin, a carbohydrate or a lectin, a complementary nucleotide sequence, an effector or a receptor molecule, an enzyme cofactor or an enzyme, an enzyme inhibitor or an enzyme, and the like. An immunoreactive specific binding member can be an antibody, an antigen, or an antibody/antigen complex that is capable of binding either to the analyte as in a sandwich assay, to the capture reagent as in a competitive assay, or to the ancillary specific binding member as in an indirect assay. If an antibody is used, it can be a monoclonal antibody, a polyclonal antibody, an antibody fragment, a recombinant antibody, a mixture thereof, or a mixture of an antibody and other specific binding members. The details of the preparation of such antibodies and their suitability for use as specific binding members are well known to those in the art.

The heterologous polypeptide produced by the present invention can be the signal generating compound (label) of the indicator reagent which is capable of generating a measurable signal detectable by external means. The various signal generating compounds (labels) contemplated include chromagens; catalysts such as enzymes for example, superoxide dismutase, horseradish peroxidase, alkaline phospatase, and β-galactosidase. The selection of a particular label is not critical, but it will be capable of producing a signal either by itself or in conjunction with one or more additional substances. A variety of different indicator reagents can be formed by varying either the label or the specific binding member. In particular, the determination of various other immunoassay formats can be followed according to the present invention. Such immunoassay system formats include, but are not intended to be limited to, competitive, sandwich and immunometric techniques. Generally, such immunoassay systems depend upon the ability of an immunoglobulin, i.e., a whole antibody or fragment thereof, to bind to a specific analyte from a test sample wherein a labeled reagent comprising an antibody of the present invention, or fragment thereof, attached to a label or detectable moiety is employed to determine the extent of binding. Such detectable labels include, but are not intended to be limited to, enzymes, radiolabels, biotin, toxins, drugs, haptens, DNA, RNA, liposomes, chromophores, chemiluminescers, colored particles and colored microparticles, fluorescent compounds such as aminomethylfluorescein, 5-fluoresceinyl, 6-fluoresceinyl, 5-carboxyfluorescein, 6-carboxyfluorescein, aminofluorescein, thioureafluorescein, and methoxytriazinolyl-aminofluorescein, and the like fluorescent derivatives. As described herein, the test sample can be a naturally occurring or artificially formed liquid, or an extract thereof, and includes, but is not intended to be limited to biological test samples such as whole blood, serum, plasma, urine, feces, saliva, cerebrospinal fluid, brain tissue, and the like. In addition, the test sample can be an extract of a test sample, or any derivative thereof.

The use of scanning probe microscopy (SPM) for immunoassays also is a technology to which the SDH-expressed heterologous polypeptide, or specific binding member thereof, produced by the present invention are easily adaptable. In scanning probe microscopy, in particular in atomic force microscopy, the capture phase, for example, at least one of the SDH-expressed heterologous polypeptides, or specific binding members there of the invention, is adhered to a solid phase and a scanning probe microscope is utilized to detect specific binding member complexes which may be present on the surface of the solid phase. The use of scanning tunnelling microscopy eliminates the need for labels which normally must be utilized in many immunoassay systems to detect antigen/antibody complexes. Such a system is described in pending U.S. patent application Ser. No. 662,147, which enjoys common ownership and is incorporated herein by reference.

The use of SPM to monitor specific binding reactions can occur in many ways. In one embodiment, one member of a specific binding partner (analyte specific substance which is the monoclonal antibody of the invention) is attached to a surface suitable for scanning. The attachment of the analyte specific substance may be by adsorption to a test piece which comprises a solid phase of a plastic or metal surface, following methods known to those of ordinary skill in the art. Or, covalent attachment of a specific binding partner (analyte specific substance) to a test piece which test piece comprises a solid phase of derivatized plastic, metal, silicon, or glass may be utilized. Covalent attachment methods are known to those skilled in the art and include a variety of means to irreversibly link specific binding partners to the test piece. If the test piece is silicon or glass, the surface must be activated prior to attaching the specific binding partner. Activated silane compounds such as triethoxy amino propyl silane (available from Sigma Chemical Co., St. Louis, Mo.), triethoxy vinyl silane (Aldrich Chemical Co., Milwaukee, Wis.), and (3-mercapto-propyl)-trimethoxy silane (Sigma Chemical Co., St. Louis, Mo.) can be used to introduce reactive groups such as amino-, vinyl, and thiol, respectively. Such activated surfaces can be used to link the binding partner directly (in the cases of amino or thiol) or the activated surface can be further reacted with linkers such as glutaraldehyde, bis (succinimidyl) suberate, SPPD 9 succinimidyl 3-[2-pyridyldithio] propionate), SMCC (succinimidyl-4-[N-maleimidomethyl] cyclohexane-1-carboxylate), SLAB (succinimidyl [4-iodoacetyl] aminobenzoate), and SMPB (succinimidyl 4-[1-maleimidophenyl] butyrate) to separate the binding partner from the surface. The vinyl group can be oxidized to provide a means for covalent attachment. It also can be used as an anchor for the polymerization of various polymers such as poly acrylic acid, which can provide multiple attachment points for specific binding partners. The amino surface can be reacted with oxidized dextrans of various molecular weights to provide hydrophilic linkers of different size and capacity. Examples of oxidizable dextrans include Dextran T-40 (molecular weight 40,000 daltons), Dextran T-110 (molecular weight 110,000 daltons), Dextran T-500 (molecular weight 500,000 daltons), Dextran T-2M (molecular weight 2,000,000 daltons) (all of which are available from Pharmacia, Piscataway, N.J.), or Ficoll (molecular weight 70,000 daltons (available from Sigma Chemical Co., St. Louis, Mo.). Also, polyelectrolyte interactions may be used to immobilize a specific binding partner on a surface of a test piece by using techniques and chemistries described by pending U.S. Patent applications Ser. No. 150,278, filed Jan. 29, 1988, and Ser. No. 375,029, filed Jul. 7, 1989, each of which enjoys common ownership and each of which is incorporated herein by reference. The preferred method of attachment is by covalent means. Following attachment of a specific binding member, the surface may be further treated with materials such as serum, proteins, or other blocking agents to minimize non-specific binding. The surface also may be scanned either at the site of manufacture or point of use to verify its suitability for assay purposes. The scanning process is not anticipated to alter the specific binding properties of the test piece.

D. Purification Methods

The assay reagent produced by the present invention can be used for isolation and purification of a member of a specific binding pair, such as an antibody, by immobilizing the assay reagent to a solid support material, such as in a column format, and contacting the assay reagent with a test sample. The member of a specific binding pair can be recovered from the immunobilized assay reagent by conditions which disrupt the binding interaction between the assay reagent and the member of a specific binding pair For example, such binding interaction can be disrupted by raising or lowering the pH or by the use of protein denaturants such as urea, guanidinium, hydrochloric acid, sodium dodecyl sulfate, and the like.

C. Test Kits

The present invention additionally provides a test kit for use in detecting the presence of a member of a specific binding pair in a test sample which test kit comprises a container containing at least one SDH-expressed heterologous polypeptide produced according to the present invention

The heterologous polypeptide can be produced as a fused or unfused protein. The expression vectors described according to the present invention are capable of expressing SDH heterologous polypeptide fusions at high levels. Example 7 illustrates the expression of superoxide dismutase as an SDH fusion polypeptide.

The test kit is presented in a commercially packaged form as a combination of one or more containers holding the necessary reagents, as a composition or admixture where the compatibility of the reagents will allow. It is to be understood that the test kit can, of course, include other materials as are known in the art and which may be desirable from a commercial user standpoint, such as buffers, diluents, standards, and the like.

The present invention will now be illustrated, but is not intended to be limited by, the following examples.

EXAMPLES

General Methods

Standard methods were employed for restriction endonuclease digestion, DNA ligation and other DNA manipulation techniques such as preparation of plasmid DNA, labeling DNA by nick translation and transformation of E. coli as described by Maniatis et. al., Molecular Cloning, A Laboratory Manual, Cold Spring Harbor, N.Y. (1982). Transformation of yeast was done according to Percival et al. Anal. Biochem. 163: 391 (1987). General methods used in the manipulation of yeast are described by Sherman et al, Methods in Yeast Genetics: A Laboratory Manual, Cold Spring Harbor, N.Y. (1983). T4 DNA ligase, calf intestinal alkaline phosphatase and restriction enzymes were used according to the supplier's instruction (BRL).

Reagents and Enzymes

Media for growth of bacteria and yeast were purchased from Difco, Detroit, Mich. Bethesda Research Laboratories (BRL), Gaithersburg Md.; New England Biolabs, Beverley, Mass.; or Boehringer Mannheim, Indianapolis, Ind. were the vendors for restriction enzymes, calf intestinal alkaline phosphatase (CIAP) and T4 DNA ligase. Zymolyase 60T was purchased from Miles Laboratory, Elkhart, Ind. Nick Translation kit and other reagents for Nick translations were obtained from Amersham Corporation, Arlington Heights, Ill. Reagents for detecting expression in λgt11 yeast genomic library were purchased from Bio Rad, Richmond, Calif. Phenylmethyl sulfonylfluoride (PMSF) was purchased from BRL.

Host Cell Cultures, DNA Sources. Vectors.

E. coli K12 strain TB1 is a derivative of JM83 (BRL). Yeast strain 8000-8B Accession Number 74215, Strain Saccharomyces cerevisiae, deposited Mar. 9, 1993; was provided by Dr. E. T. Young, University of Washington, Seattle, Wash. S. cerevisiae 8000-8B LAG- Accession Number 74214, Strain Saccharomyces cerevisiae, deposited Mar. 9, 1993; was isolated as a spontaneous mutant by selecting for growth on YNB minimal plates containing 2% sorbitol as carbon source. This mutant does not exhibit the unusual lag period for growth on sorbitol medium. Further characterization of the mutant determined that it was an a/a diploid.

E. coli vector pUC18 was purchased from Bethesda Research Laboratories. The λgt11 yeast genomic library was obtained from Dr. M. Snyder (Stanford University, Berkeley, Calif.). Yeast vectors pMW5 Accession Number 69249, Strain Escherichia coli, deposited Mar. 9, 1993; and pyCDE1 also referred to as pMAC561 were obtained from Dr. Benjamin D. Hall (University of Washington, Washington Research Foundation, Seattle, Wash.). Plasmid pWM510 Accession Number 69251, Strain Escherichia coli, deposited Mar. 9, 1993; was obtained from Dr. Wlodek Mandecki (Abbott Laboratories, Abbott Park, Ill.).

All organisms described herein by an accession number have been deposited at the American Type Culture Collection (A.T.C.C.), 12301 Parklawn Drive, Rockville, Md. 20852, as of Mar. 10, 1997, under the terms of the Budapest Treaty and will be maintained for a period of thirty (30) years from the date of deposit, or for five (5) years after the last request for the deposit, or for the enforceable period of the U.S. patent, whichever is longer.

Example 1

Isolation of sorbitol dehydrogenase activity in S. cerevisiae 8000-8B.

S. cerevisiae strain 8000-8B (obtained from Dr. E. T. Young, University of Washington) was grown to saturation at 30° C. in 1% yeast extract and 2% peptone ("YEP") medium containing either 2% sorbitol or mannitol as sole carbon source. Cells were pelleted by centrifugation, washed with 1 ml water and resuspended in 500 ml of buffer containing 50 mM potassium phosphate, pH 7.4, 1 mM PMSF, and 25 mM β-mercaptoethanol, hereinafter "extraction buffer". Cells were disrupted by vortexing the suspension with acid washed glass beads until cells appeared broken under the microscope. The cell extracts were then assayed for sorbitol dehydrogenase activity as described by Williamson et. al. Nature, 283: 214-216 (1980) and stained for enzyme activity as described in Fowler et. al. Biochem., 50: 635-645 (1972), except that 0.2 ml 20% sorbitol was substituted for ethanol. Only extracts of S. cerevisiae 8000-8B grown in medium with sorbitol produced blue stained sorbitol dehydrogenase activity bands when electrophoresed on non-denaturing gel. No stained bands were visible on the same gel containing extracts from mannitol grown culture. In order to determine the approximate molecular weight of sorbitol dehydrogenase, polypeptides contained in the activity stained band were extracted from the non-denaturing gel and re-electrophoresed on an SDS-PAGE gel. A unique band of apparent molecular weight of 40,000 daltons was present only in the sorbitol-induced cells.

Example 2

Generation of antibody to sorbitol dehydrogenase.

Step 1: Preparation of antigen for antibody production.

S. cerevisiae strain 8000-8B was grown at 30° C. to saturation in 250 ml. YEP medium containing 2% sorbitol. Large scale extracts were made by resuspending the cell pellet in 40 mL extraction buffer (described in Example 1, above) using the Bead-Beater (Biospec Products) to disrupt the cells according to the manufacturer's recommendations. Extracts were mixed with buffer containing 0.0625 M Tris pH 6.8, 2% SDS, 10% glycerol, 5% b-mercaptoethanol, hereinafter "sample buffer", boiled 3 minutes, and loaded at a quantity of 40 ml/lane into a 7.5% SDS-PAGE gel. The gel was stained for 5 minutes in 25% isopropanol, 10% acetic acid, 0.05% Coomassie blue and destained for 20 minutes in 5% methanol and 7% acetic acid. A portion of the gel containing protein(s) of approximately 40,000 daltons molecular weight was used to immunize rabbits. Antisera was prepared according to the method described by White et al. Cell, 39: 163-171 (1984). Rabbits were immunized with approximately 50 μg protein followed by a booster dose of 50 μg per rabbit. Subsequent booster doses of 50 g were given at 14 day intervals following the previous booster dose.

Step 2: Isolation and characterization of the antibody.

Western blot analysis was used to monitor the immune response of the rabbits. S. cerevisiae strain 8000-8B was grown in YEP medium containing either 2% glucose, mannitol or sorbitol. Cell extracts were prepared as described in Example 1 and electrophoresed on 7.5% SDS-PAGE gels and on non-denaturing gels. Gels were electroblotted onto nitrocellulose, blocked with 5% Carnation nonfat dry milk in "TBS" (150 mM NaCl, 50 mM Tris pH 8.1) and incubated with 1:100,000 dilution of the rabbit antisera obtained after the first booster dose. Filters were washed twice in TBS containing 0.05% Tween 20, and twice with TBS. Filters were then incubated with BioRad goat-anti-rabbit HRP conjugated second antibody, washed and stained (BioRad Express-Blot kit) according to manufacturer's recommendations. An immunoreactive band at approximately 40,000 daltons molecular weight appeared only in the lane with the extract of sorbitol grown cells as determined by BRL pre-stained high molecular weight protein markers. An immunoreactive band was visualized on the native gel corresponding to the position of sorbitol dehydrogenase activity.

Example 3

Isolation of S. cerevisiae sorbitol dehydrogenase gene.

Step 1: Isolation of clones 1S30 and 1S32 containing the S. cerevisiae sorbitol dehydrogenase gene.

The yeast sorbitol dehydrogenase gene was isolated from a lgt11 yeast genomic library obtained from Dr. M. Snyder (Stanford University) Clones 1S30 and 1S32 were isolated using the antibody probe approach described in Synder et al. Meth. Enzym., 154: 1070128 (1984). E. coli strain Y1090 (obtained from Dr. M. Snyder) was grown at 37° C. over night in Luria Broth containing 50 mgm/mL ampicillin and 0.2% maltose. Cells were collected by centrifugation and the pellet resuspended in 0.4 volumes 10 mM MgSO4. An aliquot of the lgt11 library containing 105 phage particles was then mixed with 200 mL of prepared Y1090 cells. The mixture was incubated at 37° C. for 20-30 minutes to allow phage absorption. LB top agarose (6.5 mL) was added to each sample and plated onto an LB plate containing 50 mgm/mL ampicillin. The plates were incubated at 42° C. until phage plaques were barely visible (approximately 3-4 hours). The plates were overlaid with nitrocellulose discs presoaked in 10 mM isopropyl thiogalactoside (BRL) and incubated at 37° overnight. The filters were then carefully lifted off the plates and floated in a petri dish containing a solution of 0.0625 M Tris pH 6.8, 2% SDS, 10% glycerol and 5% B-ME. After incubation for 2 minutes, the filters were washed 3 times in TBS. The filters were then incubated with blocking solution (5% Carnation non-fat dry milk in TBS) at room temperature for one hour. Blocking solution was discarded and the filters were then reacted overnight at 4° C. with a solution of the rabbit antiserum diluted 1:100 in blocking solution. The filters were subsequently washed and incubated with alkaline phosphatase conjugated goat-anti-rabbit antibody as described in the BioRad Express Blot Mt. Washing and staining were performed according to manufacturer's recommendations. Positive plaques were visualized as blue plaques and a total of eight positive plaques were obtained from 6×10⁵ total phage that were screened. After 4 rounds of plaque purification (as described in Maniatis et. al Molecular Cloning: A Laboratory Manual, Cold Spring Harbor, N.Y. (1982)), Five lambda clones XS30, XS31, XS32, XS35, and XS36 were further characterized.

Step 2: Construction of pS30 and pS32.

The inserts from the five lambda clones, isolated as described above, were subcloned into the plasmid vector pUC18. Phage DNA from clones λS30, λS31, λS32, λS35, and λS36 was prepared according to the method described by Helms et. al. DNA 4 (1): 39-49 (1985). The DNA was digested with EcoRI, ligated to EcoRI digested pUC18 (BRL) and transformed into E. coli strain TB1. Aliquots of the transformation mixture were plated onto LB plates containing ampicillin and X-Gal (BRL) using standard methods described in Maniatis. Plasmid DNA was isolated from several white colonies and analyzed by EcoRI digestion. The EcoRI inserts in some of the lambda clones had internal EcoRI restriction sites because the λgt11 yeast genomic library was constructed from sheared yeast genomic DNA. The subcloning of EcoRI inserts from the five lambda clones therefore gave rise to eight pUC18 subclones. The sizes of the EcoRI fragments ranged from 0.5 to 4.5 kilobases.

To demonstrate the relatedness of these subclones, the 4.5 kb EcoRI insert of pS30 was used to probe the eight subclones via Southern hybridization. Both pS30 and pS32 hybridized to the pS30 insert suggesting that the two clones were related to each other. Clones pS30 and pS32 were chosen for subsequent analysis and are considered interchangeable. The EcoRI inserts in pS30 and pS32 are 4.5 Kb and 3.0 Kb respectively.

Step 3: Identification of the sorbitol dehydrogenase gene encoded in pS30.

Strain 8000-8B was grown to log phase in YEP medium containing either 2% glucose, sorbitol or mannitol as a carbon source. Total RNA was prepared from the cell pellets according to the method described by Denis et. al., Mol. Cell. Biol., 6(11): 4026-4030 (1986). Cell pellets were disrupted with glass beads in RNA extraction buffer (0.1M NaCl, 0.1M Tris pH 7.5, 1 mM EDTA, 0.1% SDS). RNA was precipitated from the aqueous suspension following extraction of polypeptides with a mixture of phenol, chloroform and isoamyl alcohol. Ten micrograms of total RNA from the three different cultures were electrophoresed on 1.5% agarose formaldehyde gels as described in Maniatis et. al. RNA was blotted onto Hybond-N paper, prehybridized, hybridized and washed as described in the Amersham Hybond instruction manual. Radioactive probe was made using plasmids pS30, and pPYK by nick translation using the Amersham Nick Translation Kit. Plasmid pPYK obtained from D. Olson (University of Washington, Seattle) contains coding sequences for the yeast glycolytic protein pyruvate kinase which is expressed constituitively at a high basal level. Plasmid pPYK should therefore hybridize to a specific mRNA transcript (˜1.7 kb) made in the cultures grown with different sugars. The integrity of the RNA preparation was inferred from the results of Northern Blot hybridization experiments in which pPYK probe hybridized to specific ˜1.7 kilobase message expressed in all three cultures. RNA ladder (BRL) was used to determine the size of the mRNA transcripts.

Step 4: Sequence determination of the sorbitol dehydrogenase gene from yeast.

Clones pS30 and pS32 were used to generate the complete DNA sequence of the sorbitol dehydrogenase gene. Sequence data was initially obtained by 5' end labeling specific fragments of overlapping genomic clones (pS30 and pS32) and subjecting them to Maxam & Gilbert chemical degradation. This data led to a more informative restriction map enabling the cloning of the three HindIII fragments from genomic clone pS32. The approximately 0.5, 0..7 and 1.5 Kb fragments were subcloned into mp11 (M13 cloning/sequence vector). Complete sequence data was obtained for each subcloned fragment utilizing dideoxy chain terminating chemistry. Custom DNA primers for "gene walking" were employed to sequence entirely across reverse orientations of the subcloned fragments. DNA sequencing reaction products were electrophoresed on 8M urea 6% and 4% polyacrylamide gels and imaged using standard autoradiography film. The sequence of the gene is presented in FIG. 1. Analysis of the DNA sequence reveals an open reading frame that codes for a 357 amino acid polypeptide. A comparison of the deduced amino acid sequence of the yeast enzyme with the known sequence of the sheep liver enzyme revealed an 60.84% overall amino acid homology between the two polypeptides. In addition there was a strict conservation of 19 out of 22 residues of sheep liver SDH with the deduced amino acid sequence of the yeast sorbitol dehydrogenase polypeptide.

Example 4

Construction of yeast expression vectors pxS2, pxS3, pxS4 and pxS4 linker.

The assembly of promoter/terminator and regulatory sequences of the SDH gene in yeast plasmid pMW5 (obtained from Dr. B. D. Hall, University of Washington, Seattle, Wash.) is described below.

Plasmid pWM510 was first digested with TaqI followed by treatment with Klenow polymerase to generate DNA with blunt ends. The religated plasmid was transformed into E. coli TB1, clones were analyzed for the presence of pWM510 with the inactivated TaqI site. DNA from the transformant containing the construction (pWM510ΔTaq) was then treated with EcoRI, followed by Klenow polymerase; PstI linkers (BRL) were ligated to the blunt ended DNA in order to provide a unique PstI site on the plasmid.

The 0.5 kb PstI fragment containing part of the 5' promoter and coding sequences of the SDH gene was excised from pS32, prepared in Example 3, above and ligated to pWM510ΔTaq I Pst linker at the PstI site to form plasmid pAS1.

A separate analysis with TaqI/SspI identified clones with the PstI fragment in the correct orientation. pASI was digested with TaqI and SspI in order to replace the 143 bp. TaqI/PstI fragment with synthetic adapter DNA. The unique SspI site was p resent on pWM510 DNA sequences downstream of the PstI site. The adapter DNA was assembled from four separate oligonucleotides which were synthesized by the Applied Biosystem 380 A synthesizer using 5' dimethoxytrityl nucleoside β-cyanoethyl phosphoramidites. The four oligonucleotides were annealed and ligated to pASI at the TaqI/SspI sites. The sequence of the adapter DNA (SEQ ID NO:3 and SEQ ID NO:5, top and bottom strands, respectively) is presented below.

    1   5'CGACATAAAAAGCTCAATGTCTTACCGTTCATCTTTATGAAGAGATATAGT                            3'TGTATATTTTCCGAGTTACAGAATGGCAAGTAGAAATACTTCTCTATCA                      54  5'ATAAGTGGAAAAAAGAAACATCAAACAATCAACAAGAAAAAATACTAAAAAA                         3'TATTCACCTTTTTTCTTTGTAGTTTGTTAGTTGTTCTTTTTTATGATTTTTT                     106 5'AAAAATTGAAAAATATTACCATGGATCCCCGGGGTCGACTGAATAAATGAGTC                        3'TTTTTAACTTTTTATAATGGTACCTAGGGGCCCCAGCTAGCTTATTTACTCAGCGCT                159 5'GCGA3'                                                              

A unique NruI site was included at the 3' end of the adapter DNA in addition to unique cloning sites NcoI, BamHI, SmaI and SalI. Transformants containing the ligated DNA were analyzed for the presence of the adapter molecule by analysis of min-prep DNA with NruI and PstI. Plasmid pAS2, was identified as having the adapter DNA.

In a separate experiment the 3 kb EcoRI yeast DNA fragment containing the SDH gene was excised from pS32 and ligated into pWM510 Δ Taq at the EcoRI site. The PstI/NruI fragment containing the adapter DNA with unique cloning sites was obtained from pAS2, and inserted into pWM510ΔTaq S32 at the PstI and NcoI sites. This manipulation was done after the NcoI site was treated with Klenow polymerase. The transformant containing plasmid pAS3, with the adapter DNA was identified from EcoRI restriction analysis of mini prep DNA which released 1.5, 2, kb fragments. The EcoRI fragment containing the SDH expression cassette with promoter, terminator, and regulatory sequences from the SDH gene was then transferred into the yeast plasmid pMW5 at the EcoRI site. pMW5 was previously engineered to remove the unique BamHI site. The transformant containing the SDH cassette was identified by EcoRI restriction analysis of DNA from several isolates. Plasmids pXS2 and pXS3 had the correct 2 kb insert. Plasmid pXS2 was further shown to contain two SDH expression cassettes as determined by digestion with XhoI. In order to construct a yeast expression plasmid with an unique SalI site, the EcoRI fragment for pXS3 was excised and ligated to the yeast plasmid pMW5ΔBamΔSal (pMW5 re-engineered to inactivate the BamHI and SalI sites). Clones containing the plasmid pxS4 with unique NcoI, BamHI, SmaI, SalI sites were identified by restriction analysis of plasmid DNA prepared from several transformants.

The construction of pxS4 linker which contains the SDH expression cassette containing promoter/terminator and coding sequences for N-terminal amino acids of the sorbitol dehydrogenase polypeptide is presented in FIG. 2. Two oligonucleotides were synthesized by the Applied BioSystems 340A synthesizer as described previously. The two oligonucleotides when annealed and ligated into the NcoI/SalI sites of pxS4 should result in an SDH expression cassette containing coding sequences for the N-terminal eleven amino acids of the sorbitol dehydrogenase polypeptide. The NcoI site at the initiator ATG codon was also destroyed in this construction. The adapter DNA provides unique BamHI, SmaI and SalI cloning sites. The sequence of the adapter DNA is presented below.

5'CATGTCTCAAAATAGTAACCCTGCAGTAGTTCTAGGATCCCCCGGG3'(SEQ ID NO:7)

3'AGAGAGTTTTATCATTGGGACGTCATCAAGATCCTAGGGGGCCCACcT5'(SEQ ID NO:8)

Plasmid DNA pxS4 was digested with NcoI and SalI, ligated with several fold excess of the annealed oligonucleotides and transformed into E. coli TB1. Transformants were isolated on LB plates containing ampicillin at a concentration of 50 mgm/liter. DNA from several transformants was purified and restricted with NcoI. Plasmid DNA was extracted from the isolate containing plasmid without the NcoI site. Individual digests with NcoI, BamHI, SmaI, SalI and EcoRI confirmed that the isolate contained the adapter DNA. This plasmid was named pxS4 linker.

Example 5

Expression of Hepatitis delta antigen in yeast using expression vector pxS3.

Step 1: Construction of pxS3 delta.

DNA coding for the hepatitis delta antigen was obtained from pGB4-V5 (obtained from Dr. J. Taylor, Fox Chase Cancer Center, Philadelphia, Pa.). A 600 base pair SstII/SalI fragment containing most of the delta antigen sequence was first subcloned into PBSK+ (Stratagene) at the SstII/SalI sites (FIG. 3). Transformants containing the 600 base pair fragment were identified by restriction analysis of plasmid DNA with SstII and SalI. Clone pBSdelta' was identified as having the correct insert.

Synthetic adapter DNA coding for the N-terminal nine amino acid residues that are absent from the SstII/SalI fragment of the delta antigen gene was ligated to pBSdelta' at the SstI/SstII sites. The sequence of the adapter DNA (SEQ ID NO:4 and SEQ ID NO:6, top and bottom strands, respectively) is presented below.

5'CCCGGGTGCTAGCCATGGGCCGGTCCGAGTCGAGGAAGAACCGC3'

3'TCGAGGGCCCACGATCGGTACCCGGCCAGGCTCAGCTCCTTGG5'

The clone containing plasmid pBS delta was identified by restriction analyses with NcoI; the NcoI / SalI fragment was then inserted into the NcoI/SmaI sites of pxS3. The SalI end of the fragment was first blunt ended with Klenow polymerase. Restriction analysis of plasmid DNA with EcoRI identified clones containing DNA coding for the entire delta antigen polypeptide. The diagnostic EcoRI site in the delta antigen gene was used to confirm the identity of pxS3 delta.

Step) 2: Western Blot analysis of S. cerevisiae 8000-8B LAG expressing the hepatitis delta antigen from pxS3 delta,

S. cerevisiae 8000-8B LAG as described above was made competent as described by Percival et al. Anal. Biochem. 163: 391(1987), and transformed with pxS3 delta, and the transformants were isolated and repurified on YNB minimal--TRP plates. Individual transformants were grown in YNB minimal medium under selection; 2% sorbitol was added to provide both a carbon source and inducer for the SDH promoter. Cells were grown at 30° C. for 48 hours. Cell pellets were washed in distilled water and resuspended in lysis buffer (0.05M KPO4, pH 7.4, 1 mM PMSF). Cell lysates were prepared by disrupting the cells with glass beads for 90 seconds. Expression of the hepatitis delta antigen was determined by Western Blot Analysis.

Detection of hepatitis delta antigen expression by Western Blot analysis of extracts electrophoresed on SDS-PAGE is presented in FIG. 4 A strong immunoreactive band is evidenced only in lanes 2, and 3 which contain extracts of transformants expressing the hepatitis delta antigen. This data conclusively demonstrates the production of hepatitis delta antigen from plasmid pxS3.

Example 6

Use of pxS2 to express the human superoxide dismutase polypeptide in 8000-8B LAG.

Step 1: Construction of pxS2-SOD.

The source of the DNA coding for the human superoxide dismutase (SOD) gene was pSOD (obtained from J. Ostberg, M. Klass, Abbott Laboratories, Abbott Park, Ill.). The Applied Biosystem 340 synthesizer was used to synthesize seven separate oligonucleotides which were then assembled into the SOD gene using the Fok 1 method described by Mandecki et al. Gene, 94: 103 (1990). A 0.5 kb fragment containing the human SOD gene was excised from pSOD using restriction enzymes Bam HI and NcoI. Klenow polymerase was used to blunt end the sticky ends generated by Bam HI. The fragment was ligated to pxS2 at the NcoI and SmaI sites and the ligation reaction transformed into E. coli strain TB1. Transformants containing the SOD gene were identified by the restriction analysis of plasmid DNA extracted from several different transformants. Plasmids with SOD gene insert in the correct orientation were identified upon subsequent analysis with NcoI and EcoRV. Plasmid pxS2-SOD were identified as containing the SOD gene. (FIG. 5)

Step 2: SDS-PAGE analysis of the expression of human superoxide dismutase polypeptide in 8000-8B LAG transformed with pxS2-SOD.

Plasmids pxS4 and pxS2-SOD were transformed into S. cerevisiae 8000-8B LAG prepared as described above. The transformants were isolated in medium lacking tryptophan. Individual isolate of pxS4, pxS2-SOD was inoculated into 10 mls YNB liquid medium containing 2% sorbitol, 0.5% glucose, 25 mM CuSO₄, 25 mM ZnSO₄. A 200 microliter aliquot of saturated cultures were inoculated into 25 mls YNB liquid medium containing 2% sorbitol, 0.3% glucose, 25 mM CuSO₄, 25 mM ZnSO₄ and incubated at 30° C. Cell lysates from 10 mls of these cultures were further processed as described above. SDS analysis of the unfused superoxide dismutase is shown in FIG. 6. The results indicated a prominent polypeptide band that migrated as a 20 kilodalton polypeptide. Densitometric analysis of the gel determined that the human superoxide dismutase polypeptide was being expressed at 6% of the total soluble yeast polypeptide.

Example 7

Expression of human superoxide dismutase (SOD) polypeptide from yeast expression vector pXS4 linker.

Step 1: Construction of pXS4 linker SOD.

A chemically synthesized superoxide dismutase gene was provided by J. Ostberg and M. Klass (Abbott Laboratories, Abbott Park, Ill.). Clone pS30 contains ˜4.5 Kb yeast genomic fragment which codes for the entire yeast SDH gene. An NcoI fragment from pS30 containing approximately two thirds of the SDH gene was inserted into the unique NcoI site of plasmid pSOD which contains the chemically synthesized human superoxide dismutase gene. Restriction digestion with PstI was used to identify clone #68 which codes for a fusion of superoxide dismutase polypeptide with human SOD. This plasmid was used as a source of DNA for the human superoxide dismutase gene.

A 0.5 kb NcoI/BamHI fragment from clone #68 was isolated and treated with Klenow polymerase to fill in the sticky ends. This fragment was ligated to pXS4 linker which was previously blunt ended with Klenow polymerase at the SmaI and BamHI sites. DNA from several candidate clones was isolated and checked for th e presence of the SOD gene by restriction analysis with BamHI and SalI. Clone pXS4 linker SOD contained the SOD gene in the correct orientation.

Step 2: Analysis of expression of human superoxide dismutase polypeptide in yeast strain 8000-8B LAG transformed with pXS4 linker SOD.

The transformation protocol of Percival et al, supra, was used to introduce pXS4 linker and pXS4 linker SOD into 8000-8B LAG. Transformants were isolated under tryptophan selection which selects for the plasmid. Individual transformants were repurified on YNB minimal medium lacking tryptophan.

Transformants containing the two different plasmids were grown under tryptophan selection in 10 mls YNB liquid medium containing 2% sorbitol and 25 mM each of CuSO4 and ZnSO₄. Cells were incubated at 30° C. for 48 hours. Cell pellets were washed with 3 mls. distilled water, resuspended in 0.5 mL extraction buffer (0.05 M KP04 pH 7.4, 2 mM, b-mercapto-ethanol 1 mM PMSF) and vortexed with glass beads for 2 minutes. An equal volume of the extract was mixed with lysis buffer, boiled for 3 minutes and an aliquot of the extract was electrophoresed on SDS polyacrylamide gel which was then stained with Coomassie Blue. The data is presented in FIG. 8 which demonstrates a predominant polypeptide band that migrates with a molecular weight of 20 k daltons. Scans of the gels indicated that expression levels as high as 15% were achieved in 8000-B LAG expressing the human SOD polypeptide as a fusion with the yeast sorbitol dehydrogenase polypeptide.

It will be appreciated by those skilled in the art that of the embodiments of the present invention are applicable to other types of heterologous polypeptides and are intended as examples rather than limitations. Accordingly, the description of the invention is not intended to limit the invention of the particular embodiments disclosed, but it is intended to encompass all equivalents and subject matter within the spirit and scope of the invention as described above as and set forth in the following claims.

    __________________________________________________________________________     #             SEQUENCE LISTING                                                 - (1) GENERAL INFORMATION:                                                     -    (iii) NUMBER OF SEQUENCES: 9                                              - (2) INFORMATION FOR SEQ ID NO:1:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 2774 base                                                          (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: DNA (genomic)                                        -    (iii) HYPOTHETICAL: NO                                                    -     (iv) ANTI-SENSE: NO                                                      -     (vi) ORIGINAL SOURCE:                                                              (A) ORGANISM: SACCHAROMYCE - #S CEREVISIAE                                     (B) STRAIN: 800B                                                     -     (ix) FEATURE:                                                                      (A) NAME/KEY: CDS                                                              (B) LOCATION: 788..1856                                              -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                  - ATCCCTTTAT TGATAAGCTT AGTTTATCAA TTTTTTGTGA AAACTTTGAC TA - #ATGACGTA          60                                                                           - TTATTGTGGT GTGCATTACG ACCAAAAGGC GGGCTAACAA TTCCAAGGAA AA - #AAGCTGGT         120                                                                           - GTAAACCTGA CTGTTTGCCA CGGCTTTTCT TACTACTAAA TCGGTAGCAG GC - #ATGAAGGA         180                                                                           - ATCGTGAATG GTGTGACCTG AGACAAGGAG AGGCAGGCAT GAAGGCAAAC AA - #ATTACGGG         240                                                                           - GGAAGGCACA GGAGATACCC CCACTAAAGA TTACCCCACA ATCCCTTATT TT - #CTCGAGCT         300                                                                           - CCCTGCAGGA GAGTAAGTTT TAGGCACATA TAGGTGCAGA AAATAAAGTT TT - #CGCGGACA         360                                                                           - GCTACTGCCG AGTGGCAAAA TCTATGTTGA AGTTGGAAAG TTACGCCTAG CT - #TCAGGAAC         420                                                                           - CATCGGGCTT GGTTGGGGTG AATAATGAAG GAACTTCGTT CAGAAAACTG TT - #GTCTTTAC         480                                                                           - CAAGCACTTT AATGCTTTCT TCCTCTTCAA CAGTTAAGGT TCATCATATT CT - #ATTATCTC         540                                                                           - CGGTCTCGTA TCTCCTTTCT CCGGTCACAT GAAGCCTCTG TATCACCTTG CT - #AACCGCAT         600                                                                           - TTCTTCCATC TAAAGTATGT TCATTGCCAT AAGTTGCTTA CTCTCTCTTT AA - #TATATAGA         660                                                                           - AAAAAATTCG ACATATAAAA GGCTCAATGT CTTACCGTTC ATCTTTATGA AG - #AGATATAG         720                                                                           - TATAAGTGGA AAAAAGAAAC ATCAAACAAT CAACAAGAAA AAATACTAAA AA - #AAAAAATT         780                                                                           #GTT CTA GAG AAA GTC      829 AAC CCT GCA GTA                                  #Pro Ala Val Val Leu Glu Lys Val                                               #       10                                                                     - GGC GAT ATT GCC ATC GAG CAA AGA CCA ATC CC - #T ACC ATT AAG GAC CCC           877                                                                           Gly Asp Ile Ala Ile Glu Gln Arg Pro Ile Pr - #o Thr Ile Lys Asp Pro            # 30                                                                           - CAT TAT GTC AAG TTA GCT ATT AAA GCC ACT GG - #T ATC TGC GGC TCT GAT           925                                                                           His Tyr Val Lys Leu Ala Ile Lys Ala Thr Gl - #y Ile Cys Gly Ser Asp            #                 45                                                           - ATT CAT TAT TAT AGA AGC GGT GGT ATT GGT AA - #G TAC ATA TTG AAG GCG           973                                                                           Ile His Tyr Tyr Arg Ser Gly Gly Ile Gly Ly - #s Tyr Ile Leu Lys Ala            #             60                                                               - CCA ATG GTT TTA GGT CAT GAA TCA AGC GGA CA - #G GTT GTG GAA GTT GGT          1021                                                                           Pro Met Val Leu Gly His Glu Ser Ser Gly Gl - #n Val Val Glu Val Gly            #         75                                                                   - GAT GCC GTC ACA AGG GTC AAA GTT GGT GAC CG - #T GTT GCT ATT GAA CCT          1069                                                                           Asp Ala Val Thr Arg Val Lys Val Gly Asp Ar - #g Val Ala Ile Glu Pro            #     90                                                                       - GGT GTT CCT AGC CGT TAC TCT GAT GAG ACC AA - #A GAA GGG AGG TAT ACC          1117                                                                           Gly Val Pro Ser Arg Tyr Ser Asp Glu Thr Ly - #s Glu Gly Arg Tyr Asn            #110                                                                           - CTT TGC CCA CAT ATG GCA TTT GCT GCA ACT CC - #T CCA ATT GAT GGT ACT          1165                                                                           Leu Cys Pro His Met Ala Phe Ala Ala Thr Pr - #o Pro Ile Asp Gly Thr            #               125                                                            - CTT GTG AAG TAC TAT TTA TCT CCA GAA GAT TT - #C CTT GTG AAA TTG CCA          1213                                                                           Leu Val Lys Tyr Tyr Leu Ser Pro Glu Asp Ph - #e Leu Val Lys Leu Pro            #           140                                                                - GAA GGC GTC AGT TAT GAA GAG GGC GCT TGT GT - #C GAA CCC TTA TCA GTC          1261                                                                           Glu Gly Val Ser Tyr Glu Glu Gly Ala Cys Va - #l Glu Pro Leu Ser Val            #       155                                                                    - GGT GTA CAC TCT AAT AAA TTG GCT GGG GTC CG - #C TTT GGT ACC AAA GTT          1309                                                                           Gly Val His Ser Asn Lys Leu Ala Gly Val Ar - #g Phe Gly Thr Lys Val            #   170                                                                        - GTT GTA TTT GGT GCA GGT CCT GTG GGG CTT TT - #A ACT GGC GCA GTC GCC          1357                                                                           Val Val Phe Gly Ala Gly Pro Val Gly Leu Le - #u Thr Gly Ala Val Ala            175                 1 - #80                 1 - #85                 1 -        #90                                                                            - CGC GCT TTT GGT GCC ACC GAC GTC ATT TTC GT - #C GAT GTA TTC GAC AAC          1405                                                                           Arg Ala Phe Gly Ala Thr Asp Val Ile Phe Va - #l Asp Val Phe Asp Asn            #               205                                                            - AAG CTA CAG AGA GCA AAA GAT TTC GGA GCC AC - #A AAC ACT TTC AAT TCT          1453                                                                           Lys Leu Gln Arg Ala Lys Asp Phe Gly Ala Th - #r Asn Thr Phe Asn Ser            #           220                                                                - TCC CAG TTT TCC ACC GAT AAA GCC CAA GAC TT - #G GCC GAT GGG GTC CAA          1501                                                                           Ser Gln Phe Ser Thr Asp Lys Ala Gln Asp Le - #u Ala Asp Gly Val Gln            #       235                                                                    - AAG CTT TTG GGC GGA AAT CAC GCA GAT GTG GT - #G TTT GAG TGT TCA GGT          1549                                                                           Lys Leu Leu Gly Gly Asn His Ala Asp Val Va - #l Phe Glu Cys Ser Gly            #   250                                                                        - GCT GAT GTT TGC ATT GAT GCC GCT GTC AAA AC - #A ACT AAG GTT GGA GGT          1597                                                                           Ala Asp Val Cys Ile Asp Ala Ala Val Lys Th - #r Thr Lys Val Gly Gly            255                 2 - #60                 2 - #65                 2 -        #70                                                                            - ACC ATG GTG CAA GTC GGT ATG GGT AAA AAC TA - #C ACT AAT TTT CCA ATT          1645                                                                           Thr Met Val Gln Val Gly Met Gly Lys Asn Ty - #r Thr Asn Phe Pro Ile            #               285                                                            - GCT GAA GTT AGT GGA AAG GAA ATG AAA TTG AT - #T GGA TGT TTC CGT TAT          1693                                                                           Ala Glu Val Ser Gly Lys Glu Met Lys Leu Il - #e Gly Cys Phe Arg Tyr            #           300                                                                - TCA TTC GGT GAT TAT CGT GAC GCT GTG AAC TT - #G GTT GCC ACA GGA AAA          1741                                                                           Ser Phe Gly Asp Tyr Arg Asp Ala Val Asn Le - #u Val Ala Thr Gly Lys            #       315                                                                    - GTC AAT GTC AAG CCA TTG ATA ACC CAC AAA TT - #T AAA TTT GAA GAT GCA          1789                                                                           Val Asn Val Lys Pro Leu Ile Thr His Lys Ph - #e Lys Phe Glu Asp Ala            #   330                                                                        - GCC AAG GCT TAC GAC TAC AAC ATT GCC CAT GG - #T GGA GAG GTA GTC AAG          1837                                                                           Ala Lys Ala Tyr Asp Tyr Asn Ile Ala His Gl - #y Gly Glu Val Val Lys            355                 3 - #40                 3 - #45                 3 -        #50                                                                            #CGGCACTGGT          1886 G AATGAAAAGT GAATACTTTT                              Thr Ile Ile Phe Gly Pro                                                                        355                                                            - TCATGTCCAT ATATATAGAC CAATTCAAAA GCAGTAATAC TTGAAAATAA CA - #CCGAAAA         1946                                                                           - TAAAAATTTA AATAGTAGAC ACGTTTAATG ACTTAAAAAC TAACTTTTTC AT - #ATCTAAT         2006                                                                           - TTGTAAAATG GGCGGAAAAA GCTTAAGAAT ATGTTATTTT ACAAATCAGA GC - #GCTGACA         2066                                                                           - ATATAGAGAG CTATATGATA TGAGTGAGAG CAACTCTCCC GTATATGCTA AG - #AATATTG         2126                                                                           - CGCTTATTAG GATTGAAAGA TAGGATCAAT GAGGAATAAA TGTTACCCTT TT - #TTCTTAA         2186                                                                           - AATGTAAGAG GAAATTATGA AATATACTCT GATTTGTTTA TTATTGATTA AG - #AACAATA         2246                                                                           - AATAACCGCT CTGGTAGCTA CTGTACATAT AATTTGACGG CATATATTGC TC - #ATATATA         2306                                                                           - AACTACCGCA TTACTTCCAG TTGATAGATT TTTAACTCAG TTTCAGTACT GC - #CAGAACT         2366                                                                           - CTCATTCGAG ATTTTTTCTA TTTTTAGAAT AGGTAAAATT GCACTTAAAT GT - #ATAAGGG         2426                                                                           - TGTACGAAGT GAGTGCCCAG ACTGTTACTA TGACAATTAA ACTAATGTCG AT - #GACCATT         2486                                                                           - GTTTCGACAA CTCCATCTTC ATTTTCTTCA CGCGCCATAC TCGGATGAGA AA - #GAATCTT         2546                                                                           - TCTCTAACTA TACATTTCCA AACGCAATGA TCAAGAAGAA AGCTAAATGT TA - #CTTTGAG         2606                                                                           - TCAATTACTG AGACATGTCA TGGGAATATG AGGAAGAAAC AAATCTACGT GT - #ATTATAC         2666                                                                           - CCGTAACATG TAGAGTAAAT ACCATAGTTA CCTATTTACC TGTGTCGATA AA - #TGTTCAT         2726                                                                           #              2774GATA CATTGCTATT CCTCTGCAAT GGAAGCTT                         - (2) INFORMATION FOR SEQ ID NO:2:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #acids    (A) LENGTH: 356 amino                                                          (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: protein                                              -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                  - Met Ser Gln Asn Ser Asn Pro Ala Val Val Le - #u Glu Lys Val Gly Asp          #                 15                                                           - Ile Ala Ile Glu Gln Arg Pro Ile Pro Thr Il - #e Lys Asp Pro His Tyr          #             30                                                               - Val Lys Leu Ala Ile Lys Ala Thr Gly Ile Cy - #s Gly Ser Asp Ile His          #         45                                                                   - Tyr Tyr Arg Ser Gly Gly Ile Gly Lys Tyr Il - #e Leu Lys Ala Pro Met          #     60                                                                       - Val Leu Gly His Glu Ser Ser Gly Gln Val Va - #l Glu Val Gly Asp Ala          # 80                                                                           - Val Thr Arg Val Lys Val Gly Asp Arg Val Al - #a Ile Glu Pro Gly Val          #                 95                                                           - Pro Ser Arg Tyr Ser Asp Glu Thr Lys Glu Gl - #y Arg Tyr Asn Leu Cys          #           110                                                                - Pro His Met Ala Phe Ala Ala Thr Pro Pro Il - #e Asp Gly Thr Leu Val          #       125                                                                    - Lys Tyr Tyr Leu Ser Pro Glu Asp Phe Leu Va - #l Lys Leu Pro Glu Gly          #   140                                                                        - Val Ser Tyr Glu Glu Gly Ala Cys Val Glu Pr - #o Leu Ser Val Gly Val          145                 1 - #50                 1 - #55                 1 -        #60                                                                            - His Ser Asn Lys Leu Ala Gly Val Arg Phe Gl - #y Thr Lys Val Val Val          #               175                                                            - Phe Gly Ala Gly Pro Val Gly Leu Leu Thr Gl - #y Ala Val Ala Arg Ala          #           190                                                                - Phe Gly Ala Thr Asp Val Ile Phe Val Asp Va - #l Phe Asp Asn Lys Leu          #       205                                                                    - Gln Arg Ala Lys Asp Phe Gly Ala Thr Asn Th - #r Phe Asn Ser Ser Gln          #   220                                                                        - Phe Ser Thr Asp Lys Ala Gln Asp Leu Ala As - #p Gly Val Gln Lys Leu          225                 2 - #30                 2 - #35                 2 -        #40                                                                            - Leu Gly Gly Asn His Ala Asp Val Val Phe Gl - #u Cys Ser Gly Ala Asp          #               255                                                            - Val Cys Ile Asp Ala Ala Val Lys Thr Thr Ly - #s Val Gly Gly Thr Met          #           270                                                                - Val Gln Val Gly Met Gly Lys Asn Tyr Thr As - #n Phe Pro Ile Ala Glu          #       285                                                                    - Val Ser Gly Lys Glu Met Lys Leu Ile Gly Cy - #s Phe Arg Tyr Ser Phe          #   300                                                                        - Gly Asp Tyr Arg Asp Ala Val Asn Leu Val Al - #a Thr Gly Lys Val Asn          305                 3 - #10                 3 - #15                 3 -        #20                                                                            - Val Lys Pro Leu Ile Thr His Lys Phe Lys Ph - #e Glu Asp Ala Ala Lys          #               335                                                            - Ala Tyr Asp Tyr Asn Ile Ala His Gly Gly Gl - #u Val Val Lys Thr Ile          #           350                                                                - Ile Phe Gly Pro                                                                      355                                                                    - (2) INFORMATION FOR SEQ ID NO:3:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 162 base                                                           (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: DNA (genomic)                                        -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                  - CGACATATAA AAGGCTCAAT GTCTTACCGT TCATCTTTAT GAAGAGATAT AG - #TATAAGTG          60                                                                           - GAAAAAAGAA ACATCAAACA ATCAACAAGA AAAAATACTA AAAAAAAAAA TT - #GAAAAATA         120                                                                           # 162              GGGT CGACTGAATA AATGAGTCGC GA                               - (2) INFORMATION FOR SEQ ID NO:4:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 45 base                                                            (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: DNA (genomic)                                        -    (iii) HYPOTHETICAL: YES                                                   -     (vi) ORIGINAL SOURCE:                                                              (A) ORGANISM: SACCHAROMYCE - #S CEREVISIAE                           -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                  #45                TGGG CCGGTCCGAG TCGAGGAAGA ACCGC                            - (2) INFORMATION FOR SEQ ID NO:5:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 160 base                                                           (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: DNA (genomic)                                        -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                  - TCGCGACTCA TTTATTCAGT CGACCCCGGG GATCCATGGT AATATTTTTC AA - #TTTTTTTT          60                                                                           - TTTGATATTT TTTCTTGTTG ATTGTTTGAT GTTTCTTTTT TCCACTTATA CT - #ATATCTCT         120                                                                           #   160            TAAG ACATTGAGCC TTTTATATGT                                  - (2) INFORMATION FOR SEQ ID NO:6:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 43 base                                                            (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: DNA (genomic)                                        -    (iii) HYPOTHETICAL:  YES                                                  -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                  # 43               CCGG CCCATGGCTA GCACCCGGGA GCT                              - (2) INFORMATION FOR SEQ ID NO:7:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 46 base                                                            (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: DNA (genomic)                                        -    (iii) HYPOTHETICAL: YES                                                   -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                  #                 46ACC CTGCAGTAGT TCTAGGATCC CCCGGG                           - (2) INFORMATION FOR SEQ ID NO:8:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 48 base                                                            (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: DNA (genomic)                                        -    (iii) HYPOTHETICAL:  YES                                                  -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                  #                48TAGA ACTACTGCAG GGTTACTATT TTGAGAGA                         - (2) INFORMATION FOR SEQ ID NO:9:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 46 base                                                            (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: DNA (genomic)                                        -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                  #                 46CCG GGGTCGACTG AATAAATGAG TCGCGA                           __________________________________________________________________________ 

What is claimed is:
 1. A DNA segment comprising, in a 5' to 3' direction: (i) a regulatory sequence of the Saccharomyces cerevisiae sorbitol dehydrogenase gene wherein said regulatory sequence comprises the promoter of said sorbitol dehydrogenase gene, operably linked to (ii) a DNA sequence coding for the N-terminal eleven amino acid residues of the sorbitol dehydrogenase polypeptide, (iii) a gene coding for a heterologous polypeptide, and (iv) a terminator of said sorbitol dehydrogenase gene, whereby expression, in Saccharomyces cerevisiae, of a vector comprising said DNA segment results in a polypeptide which is a fusion of the peptides encoded by (ii) and (iii).
 2. The DNA segment of claim 1 wherein said promoter includes nucleic acid numbers 1-771 and said terminator includes nucleic acid numbers 1848-2760 of FIG.
 1. 3. The DNA segment of claim 1 wherein said DNA sequence encoding the N-terminal eleven amino acid residues of the sorbitol dehydrogenase polypeptide comprise nucleic acid numbers 772-805 of FIG.
 1. 4. The DNA segment of claim 1 wherein the heterologous polypeptide is superoxide dismutase.
 5. The DNA segment of claim 1 wherein the heterologous polypeptide is hepatitis delta antigen.
 6. A Saccharomyces cerevisiae host cell transformed with an expression vector comprising the DNA segment of claim
 1. 